1) Field of the Invention
The present invention generally relates to an image formation apparatus including a toner image carrying body such as a photosensitive drum, a dielectric drum or the like to which a toner image obtained from toner development of an electrostatic latent image is electrostatically adhered and held, and in particular, relates to a toner image transferring device incorporated therein for electrostatically transferring the toner image from the toner image carrying body to a sheet of paper.
2) Description of the Related Art
As a representative of an image formation apparatus as mentioned above, an electrophotographic recording apparatus is well known, wherein the following processes are typically carried out:
a) a uniform distribution of electrical charges is produced on a surface of an electrostatic latent image carrying body; PA1 b) an electrostatic latent image is formed on a charged area of the body surface by an optical writing means such as a laser beam scanner, an LED (light emitting diode) array, a liquid crystal shutter array or the like; PA1 c) the latent image is developed as a visible image with a developer or toner, which is electrically charged to be electrostatically adhered to the latent image zone; PA1 d) the developed and charged toner image is electrostatically transferred from the body to a recording medium such as a cut sheet paper; and PA1 e) the transferred toner image is fixed and recorded on the cut sheet paper by a toner image fixing means such as a heat roller.
Typically, the electrostatic latent image carrying body may be an electrophotographic photoreceptor, usually formed as a drum, called a photosensitive drum, having a cylindrical conductive substrate formed of a metal such as aluminum, and a photoconductive insulating film bonded to a cylindrical surface thereof and formed of an organic photoconductor (OPC), a selenium photoconductor or the like.
In the transferring process, the transfer of the toner image to the paper is carried out by using a transfer charger to feed an electric charge to the paper. In particular, the transfer charger is disposed in the vicinity of the photosensitive drum, the paper is introduced into a clearance therebetween, and the transfer charger gives the paper an electric charge having a polarity opposite to that of the electric charge of the toner image, whereby the toner image is electrostatically transferred from the drum to the paper. For example, the transfer charger may be a corona discharger.
To increase the efficiency of the transfer of the toner image, the paper must be given a sufficient amount of electric charge. Of course, the transfer charger is designed so as to be provided with a capacity needed to give the paper the sufficient amount of electric charge, but the transfer charger must be properly positioned with respect to the photosensitive drum before a desired efficiency of the transfer of the toner image can be obtained.
During the transferring process, the paper is adhered to the photosensitive drum due to an electrostatic attraction acting therebetween, and thus the paper can often be entangled with the drum. Thus, conventionally, for example, a pair of wedge-shaped separating members are provided at an exit of the clearance between the photosensitive drum and the transferring device to be engaged with a surface of the drum at the side edges thereof, so that the leading edge of the paper is mechanically separated therefrom. Alternatively, the transferring device includes an AC charge eliminator integrally associated with the transfer charger. The AC charge eliminator may comprise a corona discharger which is energized with an alternating current, whereby the charge is partially eliminated from the paper to which the toner image has been transferred. Thus, the electrostatic attraction between the photosensitive drum and the paper is weakened so that the paper can be easily separated from the drum, but the AC charge eliminator must be properly positioned with the drum before the separation of the paper from the drum can be surely ensured.
Conventionally, the association of the transfer charger with the AC charge eliminator involves an incompatibility in that the respective proper positions therefor interfere with each other. Namely, for example, when the transfer charger is properly positioned with respect to the photosensitive drum, the ability of the AC charge eliminator cannot be exhibited, and vice versa.
Also, an area or clearance between the toner image transferring device and the photosensitive drum must be kept constant before a stable transferring process can be ensured. Nevertheless, it is difficult to maintain the clearance between the toner image transferring device and the photosensitive drum because the drum cannot have a perfect circular profile. Accordingly, although the transferring device is positioned in place with respect to the drum, the clearance therebetween varies during the rotation of the drum. To maintain the clearance between the transferring device and the drum, the transferring device is provided with spacer rollers, and is resiliently biased toward the drum so that the spacer rollers thereof are resiliently pressed against a rotating surface of the drum, to thereby keep the clearance therebetween constant. To this end, conventionally, a printing unit including the photosensitive drum and the transferring device is frequently constituted as an integrated unit. However, this integrated unit is undesirable considering maintenance thereof, because manual access to an interior of the integrated unit is very difficult. Also, when a jamming of paper in the integrated unit occurs, removal of the jammed paper therefrom is very troublesome.